A battery system includes battery cells, e.g., Lithium ion cells, for powering cell phones, laptop computers or electric vehicle. A battery balancer is widely used to maximize the capacity of the battery cells and increase the battery's lifetime. If the battery cells experience an unbalanced condition, then the battery balancer balances the battery cells by drawing energy from the most charged cell or by drawing energy from the most charged cell to the least charged cell.
FIG. 1 shows waveforms 100 of a cell voltage and a charging current associated with a Lithium ion battery cell during charging. The Lithium ion battery cell is charged in multiple charging modes such as a constant current (CC) charging mode and a constant voltage (CV) charging mode. At time t0, the Lithium ion battery cell has a voltage level V10, and a charging current is provided to charge the battery cell. During the time interval between t0 and t3, the battery cell operates in the constant current mode in which the charging current is constant and the voltage increases from the level V10 to the level VM. During the time interval between t3 and t4, the battery cell enters a constant voltage mode in which the charging current gradually decreases and the cell voltage is maintained at a constant level. For example, the voltage level of the battery cell is maintained at VM during time t3 and time t4. At time t4, the charging current decreases below a current threshold. In response, the battery charging is terminated.
During the constant current charging mode, there is a time period in which the cell voltage rises at a relatively low rate (the time period is called a flat voltage region hereinafter). For example, the rising rate of the cell voltage between t1 and t2 is less than a rate threshold. As the charging current is constant, the rate at which the capacity of the battery cell increases is supposed to be the same at any time in the constant current mode. Thus, the cell voltage in the flat voltage region, e.g., having a relatively low rising rate, does not accurately reflect the variation in the cell capacity.
Therefore, for a battery pack that includes multiple battery cells, the battery balancer may perform the balancing operation when all the battery cells operate in the flat voltage region. Since the cell voltages do not reflect the capacity of a battery cell, it is a challenge for the battery balancer to detect whether the unbalanced condition is corrected by monitoring the cell voltages of the battery cells. For example, although energy of the most charged cell is drawn to the least charged cell during balancing, the voltages of the cells do not vary in response to the variation of cell capacities. Thus, the balancing control of the battery balancer may not be accurate.